A fuel cell is a device that directly converts the chemical energy of reactants (a fuel and an oxidant) into low-voltage d.c. electricity. Many of the operational characteristics of fuel cell systems are superior to those of conventional power generation. Among several distinct types of fuel cells, the polymer electrolyte membrane or proton exchange membrane (PEM) fuel cell is most popular for transportation and portable applications. The PEM fuel cell could employ compressed hydrogen gas or methanol reformate as fuel. Other hydrocarbons, such as gasoline, diesel fuel, or ethanol could also be reformed to produce suitable reformate for the fuel cell (U.S. Pat. No. 5,928,614 of Autenrieth and Heil, U.S. Pat. No. 4,865,624 of Okada, U.S. Pat. No. 5,984,986 of Weisheu et al., U.S. Pat. No. 5,651,800 of Mizuno et al., U.S. Pat. No. 4,909,808 of Voecks, and U.S. Pat. No. 5,484,577 of Buswell et al.). Although a fuel cell operating on pure hydrogen gas is considered to be the ultimate clean energy system, the difficulties associated with handling high-pressure compressed hydrogen gas and the lack of a hydrogen infrastructure would prevent the mass use of the fuel cell power system in the foreseeable future. As a result, fuel cell power systems using reformate from methanol or from other hydrocarbons such as gasoline are actively under development. One of the drawbacks for reformate based fuel cell power systems is that a large amount of energy is needed for the fuel processing purpose. The total heat energy requirement for a reformer can be estimated by using the following relation (U.S. Pat. No. 5,997,594 of Edlund and Pledger):ΔHtot=ΔHrxn+ΔHvap+ΔHcp+ΔHlosswhere ΔHrxn is the enthalpy of reforming reaction; ΔHvap is the enthalpy of vaporization of the liquid feedstock; ΔHcp is the enthalpy required to heat the vaporized feedstock to the reforming temperature; and ΔHloss is the heat lost to the ambient which could be minimized with adequate insulation. It was estimated that heating value equivalent to that of about 20% to 30% of the hydrogen produced in the reformer is needed to provide a fuel stream with sufficient heating value to meet the heating requirement, ΔHtot, of the reformer. This amount of heating value is usually provided through the combustion of remaining hydrogen/hydrocarbons in the exhaust gases from the fuel cell anode, burning the hydrogen/hydrocarbons in the byproduct stream of the reformer, or consumption of additional hydrocarbon fuel other than that being reformed in the reformer. It is evident that the energy input to the reformer must be reduced if the efficiency of a fuel cell power system is to be increased.
Another problem generally associated with a PEM fuel cell power system is the difficulty in dissipating the waste heat generated by the fuel cell stack. The voltage efficiency of a PEM fuel cell stack under normal operating condition is about 50 to 70%. This means that 30 to 50% of the energy content of the hydrogen participating in the electrochemical reaction in the fuel cell stack will be dissipated into waste heat that must be removed from the fuel cell stack under steady state operating condition. Since a PEM fuel cell normally operates within a temperature range of 60-80° C. that is substantially lower than that of an internal combustion engine, a cooling system employing conventional radiators would require much more space and fan power for adequate heat removal from the fuel cell stack. The present invention has been made to overcome these difficulties described above.